Problems in a utility network, or “faults”, (such as network device failures) can affect how power is distributed throughout the network. In particular, faults tend to drain energy from power sources, leaving less energy for distribution throughout other areas of the network and for recovering from voltage “sags” resulting from the fault.
When a fault occurs in a utility network, momentary voltage depressions are experienced, which may result in voltage collapse or voltage instability on the network.
In general, such a fault appears as an extremely large load materializing instantly on the utility network. In response to the appearance of this load, the network attempts to deliver a large amount of current to the load (i.e., the fault). Detector circuits associated with circuit breakers on the network detect the over-current situation immediately (within a few milliseconds). Activation signals from the detector circuits are sent to protective relays which initiate opening of the circuit. The mechanical nature of the relays generally requires 3 to 6 cycles (i.e., up to 100 milliseconds) to open. When the breakers open, the fault is cleared.
Power cables using high temperature superconductor (HTS) wire are being developed to increase the power capacity in utility power networks while maintaining a relatively small footprint. Among other advantages, the HTS power cables are much easier to site, even in dense, older urban areas. Such HTS cables also allow larger amounts of power to be pumped economically and reliably into congested areas of a utility power network and transferred precisely where it is needed to relieve congestion. An HTS power cable uses HTS wire in the core of the cable instead of copper for the transmission and distribution of electricity. The design of HTS cables results in significantly lower impedance compared to conventional lines and cables. The use of HTS wire enables a three to five times increase in current-carrying capability compared to alternating current (AC) conventional cables, and up to ten times more power flow through direct current (DC) conventional cables.
HTS power cables behave differently than conventional non-superconducting cables to fault currents. First, a cold dielectric HTS power cable requires that the cooling liquid must remain in a sub-cooled state during a major fault or multiple through faults. This is necessary to maintain the dielectric strength between the high voltage cable core and the shield, which is at ground potential. Any bubble formation inside dielectric will threaten the dielectric properties of the insulation. Second, the cable must be off line following major faults in order to allow enough time for the HTS conductors to be cooled back down to the operating temperature range. As a result, conventional cable fault protection schemes are not suitable for use with HTS power cables.